Plate 100

Pebble-made groove casts

These marks were photographed on a detached block of pebbly sandstone, and have a very pronounced relief. Rounded groove casts are recognizable, part of which has a sinuous trend. They end in, or are overprinted by bulbous, lobate mounds that conceal pebbles, visible within the bed in sections (very few of them are preserved on the basal surface). The weight of the pebbles made the sand sink into the mud; this load effect is responsible for the roughness of the surface.

The sandstones are not classical turbidites (as were most examples presented earlier) but are rather proximal gravity flow deposits of a fan delta. The closeness of a fluvial source explains the abundance of pebbles.

Colombacci Formation, Pietrarubbia, northern Apennines.